PSI - Issue 57

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Lewis Milne et al. / Procedia Structural Integrity 57 (2024) 365–374 Lewis Milne et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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Equation 2 was applied along the length of the UFT SN curve for both Q355B and S355JR. The resulting curves, plotted in yellow in Figure 8, showed similar trends to the corrected curves produced using the average discrepancy method. It matched well with the S355JR data, but not with the Q355B data, due to the gradient of the ultrasonic results being markedly different to that of the conventional frequency results. As such, this model cannot yet be considered accurate for relating results at different test frequencies. 10. Conclusions In this paper, the fatigue results for two equivalent grades of structural steels, Q355B and S355JR were evaluated at conventional 20Hz test frequencies and at 20kHz test frequencies. Several different methods were applied to try and evaluate the frequency sensitivity of the materials and to relate the UFT data to the conventional frequency data. The following observations were made: • Both S355JR and Q355B steels exhibited similar fatigue properties at ultrasonic frequencies, with a slightly higher fatigue resistance observed for Q355B due to the higher yield strength of the particular metal batch tested • In all cases, failure originated from the surface of the specimens • Due to the use of UFT geometry for conventional frequency testing of Q355B, the fatigue resistance was observed to be much higher than that of S355JR, however it also increased the amount of scatter around the fatigue limit • Two comparison methods based on the finite life fatigue region were applied to the results for both materials • Corrected SN curves produced using both comparison methods correlated well with the S355JR data, but not with the Q355B data and were therefore not considered to be reliable • The discrepancy between the survivable stress amplitudes at the two frequencies was 1.32x higher for S355JR, which was which was tested with at conventional frequencies using a risk volume which was approximately 17x larger than the Q355B. This provides an estimate of the influence of the size effect • Literature results on similar steels mostly use larger specimens at conventional frequencies, and report similar frequency sensitivities as the S355JR in this investigation. This suggests that some results in literature may overestimate the influence of frequency on fatigue results due to the influence of size effects • Applying models across different investigations in literature is challenging because of the lack of consistency in test parameters Acknowledgements The authors would like to acknowledge the support for this study, which was provided by the Weir Group PLC (WARC2011-SAA1, 2011) via its establishment of the Weir Advanced Research Centre (WARC) at the University of Strathclyde. References ASTM International, 2020. ASTM E562-19: Standard Test Method for Determining Volume Fraction by Systematic Manual Point Count . West Conshohocken. ASTM International,2022. ASTM A370-22: Standard Test Methods and Definitions for Mechanical Testing of Steel Products . Bach, J., M. Stoiber, L. Schindler, H. W. Höppel, and M. Göken, 2020. Deformation Mechanisms and Strain Rate Sensitivity of Bimodal and Ultrafine-Grained Copper. Acta Mater 186 : 363 – 73. Bach, J., M. Göken, and Heinz-Werner Höppel, 2018. Fatigue of Low Alloyed Carbon Steels in the HCF/VHCF Regimes, In “ Fatigue of Materials at Very High Numbers of Loading Cycles, ” H. -J. Christ (Ed.). Springer Fachmedien Wiesbaden, pp. 1 – 23. Bathias, C., 1999. There Is No Infinite Fatigue Life in Metallic Materials. Fatigue Fract Eng Mater Struct 22 (7): 559 – 65.